MX2012014452A

MX2012014452A - Method of deployment, method and device for seismic prospecting in an aquatic medium.

Info

Publication number: MX2012014452A
Application number: MX2012014452A
Authority: MX
Inventors: Michel Manin
Original assignee: Kietta
Priority date: 2010-06-10
Filing date: 2011-06-10
Publication date: 2013-02-26
Also published as: US9529107B2; WO2011154545A3; GB201300208D0; CA2801294C; BR112012030767A2; AU2011263645A1; GB2495655B; NO20130038A1; FR2961317A1; WO2011154545A2; GB2495655A; US20130201790A1; NO345753B1; CA2801294A1; AU2011263645B2; WO2011154545A4; CN103052893A; FR2961317B1

Abstract

There is proposed a method of seismic prospecting in an aquatic medium with the aid of a device comprising at least one seismic cable furnished with sensors and at least one mobile seismic source. The method comprises the unfurling of the cable in the water, constrained by a value of maximum curvature of track in the water and by a maximum value of deviation with respect to a desired course in the terrestrial reference frame, the course being subject to a maximum value of speed with respect to the terrestrial reference frame. The method comprises, simultaneously, the displacement of the seismic source in a reference frame tied to the cable and the emission of waves by the seismic source, the waves being reflected by the aquatic bottom and sensed by the cable. Such a method makes it possible in particular to obtain a good seismic density above a field under investigation during seismic prospecting, with lower motorization loads and lower stresses exerted on the cable.

Description

DEPLOYMENT PROCEDURE. PROCEDURE AND DEVICE FOR SEISMIC PROSPECTION IN MEDIUM WATER Field of the Invention The present invention relates to the field of seismic prospecting in aquatic environment (marine or lacustrine).

Background of the Invention Procedures are known which consist of placing a series of seismic cables (or lines or "streamer" in English) parallel, submerged, kept laterally by paravans, in each of which are placed spaced sensors of the hydrophone and / or geophone type, being the cables towed by one or more boats.

Another (or several) named boat (source), equipped with means capable of creating a wave in a marine environment, generally in the form of an air cannon, is displaced remotely from the sensor cables. The waves thus formed are propagated to the seabed, and then in the different geological layers to be reflected by the latter, and finally to be collected and measured by such submerged sensors. The source ship can be a boat that pulls the seismic cables.

The set of information is then processed to make a three-dimensional (3D) image of the different geological layers of the submarine subsoil, so general used to determine the possible presence of petroleum deposits.

This technique has been used for many years, and is subject to very strict application requirements. First, the dynamic noise due to the towing of the cables disturbs the measurement of the waves to be collected. In addition, the hydrodynamic resistance resulting from the resistance of the cables is very high, and is counted in tens of tons, for example in the order of 70 tons, which leads to use very powerful traction boats. This is especially due to the speed in the water required for the procedure and the presence of paravans, which create a resistance. In addition, the weight and induced hydrodynamic resistance cause the traction cable of the paravans to experience a dynamic deformation effect of the "piano rope" type during towing. This leads to cable fatigue and may involve its breaking. Extremely high substitution costs can be derived, taking into account the immobilization of the device as a whole. In addition, in classic procedures, the cables must be submerged at a shallow depth, between 5 and 10 meters, which entails an accident risk taking into account the circulation of surface vessels that have a strong draft (tankers or container ships) and a strong sensitivity to the state of the sea.

On the other hand, known seismic survey devices leave shaded areas during the measurement. Indeed, the cables are generally about 8 km long and spaced approximately 100 m apart, which leads, for a dozen parallel cables, to a measuring area of 1 x 8 km. Now, the ideal from the point of view of the measurement is the use of an isotropic system, that is to say a square surface, for example of 8 x 8 km. However, these dimensions are incompatible with the towing means that would be necessary, with respect to the weight, of the resistance and the logistics necessary to obtain such measuring surface.

Therefore, an attempt has been made to remedy this situation in two known ways.

The first attempt (called "Wide Azimuth") consists of replacing the anisotropy with the use of one (or two) boats that tow a set of cables that form a measuring area of 1 x 8 km, and using 2 to 8 source boats. This device has two main drawbacks. First, the prohibitive cost resulting from the investment in materials, maintain and use (2 to 8 source boats, plus one (or two) towed boats, plus the set of cables). The other drawback lies in the fact that the source boats (pull) (ie, emit waves) one after another, and therefore two to eight times less, which leads to a very low draft density.

The second attempt proposed in a known manner is shown by the patent application GB No. 2 435 931, in the name of Western Geco, which describes a method and device consisting schematically in a network of sensors (geophones) fixed to a structure of two. dimensions (in the form of a mesh or network) or three dimensions. The structure presents a periphery (perimeter or envelope) kept in shape by dynamic means such as drones or small boats, to keep in shape the mesh that constitutes the structure. The latter is towed continuously and one or more seismic sources are foreseen.

Despite the apparent attraction, in the theoretical plate, of the device and the method thus proposed, it is still difficult for this device to be realistically implemented. In effect, the structure constituted in this way would present a huge weight and resistance and would force the use of means to keep in shape, excessive and out of regulation both at the technical level, as well as at the financial or budgetary level. In addition, it only offers a construction with only one possible geometry for the sensor network.

According to another aspect, in general, the marine seismic survey aims to capture or recover the maximum of signals to make a geological mapping as accurate and reliable as possible of the underlying areas of the submarine bottom. However, low frequency signals provide information about very deep deposits, and are therefore valuable in this regard. However, low frequency signals are strongly attenuated by the phenomenon of surface reflection, called "phantom" and partly due in particular to the fact that the cable, according to the prior art, is submerged a few meters from the surface. In this way, it is sought to eliminate these "ghosts" to obtain what is called a "flat spectrum". An attempt has been made to remedy this situation by using a technique known as "over-under", which consists in arranging two cables carrying hydrophone sensors, one under the other vertically, at depths for respective examples of 20 m and 26 m. The treated combination of the two signals received by the two respective cables allows to mitigate even eliminate, the consequences of "ghosts". However, this known method, in addition to the additional treatment it needs, has the main drawback of greatly reducing productivity and increasing costs, due to the duplication of cables and sensors.

Another known technique that seeks to eliminate the "ghosts", proposed by the company PGS, consists of using lines or cables that carry, in addition to hydrophones (which measure pressure), geophones or accelerometers capable of measuring the speed or acceleration of the wave. As the reflection coefficients for the respective measurements of pressure (hydrophones) and velocity (geophones) inverses (-1 and +1), it is thus theoretically possible to cancel the "phantoms". This known technique has the disadvantages of requiring a high investment in sensors and generating an annoying noise at the level of the geophones or accelerometers that is derived from the traction speed (of approximately 5 knots) generating parasitic vibrations.

The invention proposes to remedy, at least in part, the drawbacks mentioned above.

To this end, the present invention proposes, according to a first aspect, a seismic survey procedure in an aquatic environment with the aid of a device comprising at least one seismic cable equipped with sensors and at least one mobile seismic source. The procedure comprises the steps that consist of moving the cable in the water, and, simultaneously, moving the seismic source in a referent linked to the cable, emitting waves through the seismic source, and capturing reflections of the waves by the cable. The displacement of the cable minimizes the deviation of the cable with respect to a desired route in the land reference. The displacement of the cable is also limited by a maximum curvature value of travel in the water. In other words, the displacement of the cable in the water is defined by a program of minimization of the deviation of the cable with respect to the desired route, having as limitation the maximum curvature value of travel in the water.

According to another aspect, the invention proposes a deployment procedure (i.e. displacement) in aquatic environment of a device comprising at least one seismic cable provided with sensors. The method comprises a step consisting in moving the cable in the water. As in the seismic survey procedure, the displacement of the cable minimizes the deviation of the cable with respect to a desired route in the terrestrial part and is also limited by a maximum curvature value of travel in the water.

The invention also proposes a seismic survey device, for example such as that implemented in the seismic survey procedure or the deployment procedure. The device comprises at least one cable provided with sensors, and a calculation unit for determining the displacement of the cable in the water. The calculation unit calculates the displacement of the cable that minimizes the deviation with respect to a desired route in the terrestrial part, the displacement of the cable being furthermore limited by a maximum curvature value of travel in the water. In other words, the calculation unit can solve a program for minimizing the deviation of the cable with respect to the desired route, having as a limitation the value of the maximum curvature of travel in the water.

The cable can also, in an appropriate manner, be provided with two drones each attached to one end of the cable. In this case, the drones can put the cable under tension and put it in motion in the aquatic environment, exerting a tension force on the cable. In this way the expression "motor" can be used to designate the drone that exerts the dominant tension force. In other words, the motor drone puts the cable in motion "towing" it. Other features and advantages of the invention will be apparent from the following description of a preferred embodiment mode of the invention, offered by way of example and with reference to the accompanying drawings which show: Figure 1 represents a scheme of prospecting device; Figures 2-4 represent trajectory trajectories for a fixed route; Figure 5 represents the theoretical path of a cable subjected to the current of Figure 4 for a fixed route; Figures 6-7 represent the displacement of a cable with a fixed desired route; Figures 8-13 represent the evolution of a cable with a fixed desired path in the case where current predictions are available; Figures 14-17 represent the displacement of a cable with a fixed desired route in real time; Figure 18 represents a bottom view of the cables of the device of Figure 1 and a line followed by a seismic source; Y Figure 19 shows an example of a cable displacement control loop.

A seismic survey procedure in an aquatic environment, for example marine or lacustrine, can be implemented with the aid of a device comprising at least one seismic cable provided with sensors and at least one seismic source, positioned in the water to allow the prospecting of a certain area of the subsoil. The procedure comprises the displacement of the cable in the water, and, simultaneously, the emission of waves by the seismic source, preferably submerged, which allows noise to be reduced. The waves, for example acoustic waves, are reflected in the subsoil by the interfaces between the geological layers of the area mentioned above, as well as by the bottom of the aquatic media and captured by the cable.

Figure 1 shows an example of such a seismic survey device 100. The mobile seismic source is not shown in figure 1. The seismic source is capable of creating a disturbance transmitted by the aquatic environment in the form of waves. The device 100 comprises at least one cable 110, and preferably several, provided with a plurality of seismic sensors (for example hydrophones) capable of collecting such reflected waves. Such a cable can be called "seismic cable", or also "seismic flute". The cable 110 evolves in a measuring station adapted for the prospecting of a part of the aforementioned area of the subsoil. Typically, to perform the procedure, such a seismic source is activated. It is captured with the aid of such sensors 106 of such reflected waves. It moves to another measuring station adapted for prospecting another part of the aforementioned area, on the one hand the cable 110 and on the other hand the seismic source, and so on.

Cable 110 evolves in water. In general, it can be motionless, that is to say, drifting, or it can move in the water. The seismic source moves, during the prospecting procedure, in a reference linked to the cable. It is understood with this that the seismic source moves globally with respect to the cable. This allows to increase the number of measurements in a shorter time. You can take, for example, a reference whose origin is one end of the cable, or the center of the cable, and whose axes are orthogonal, one of the axes being in the direction tangent to the cable in this origin. The seismic source is located at a distance from the cable that allows capture by the cable of the waves emitted and then reflected by the geological layers of the subsoil and the bottom of the aquatic environment.

The evolution (the term "evolution" can designate "displacement" in the following) of the cable 110 is limited by a maximum curvature value of travel in the water and by a maximum value of deviation with respect to a desired route in the terrestrial reference. Appropriately, the route is subject to a maximum speed value with respect to the land reference.

By "path" is meant a set of pairs (u, t) where u represents a position in the water reference and t a moment, following the successive positions of a curve corresponding to a parameterized arc whose parameter is time. The set can be discrete in time, separating a time stage then two successive positions, or being continuous in time (the path is then the parameterized arc mentioned above). The cable 110 can optionally move in the water reference. When this is not specified, the route refers to positions given in the water reference. In the present case, the path of the cable 110 is understood with respect to the water reference.

The path thus defined corresponds to the displacement of a point. However, it is said by extension that the cable 110 evolves in the water according to the route since if one of its ends A or B is in motion, it is considered that the rest of the cable 110 is dragged in the groove and therefore follows the same route in the water reference. The cable 110 therefore moves according to its axis. If the cable is on the contrary to drift, we can speak of null travel, since the path is reduced to a fixed point in the water reference. The curvature of the path is the curvature of the parameterized arc mentioned above corresponding to the path, taking the classic definition of the curvature of a parameterized arc.

By "limiting" it is meant that the method ensures that the cable can not follow a path having a curvature greater than the maximum curvature value, and that the cable does not deviate from the desired route beyond the maximum deviation value. Eventually, the method may then comprise a verification step which ensures that these limitations are respected and a correction step if necessary.

Limiting the evolution of the cable 110 by a maximum value of curvature allows to reduce the energy costs and the mechanical stresses experienced by the cable. On the other hand, the aquatic current (for example the marine current if it is in an aquatic environment) can be considered as homogeneous in the length of the cable 110 at a given moment of evolution. Limiting the path with a maximum curvature value then allows a maximum radius to be imposed on the cable 110. This allows to avoid an excessive disturbance of the geometry of the cable 110 and to preserve an effective use length of the cable 110 during the measurements and thus obtain a better seismic density (ie, distribution of the reflection points of the waves on the cable 110).

The evolution of the cable 110 is also limited by a maximum value of deviation with respect to a desired route in the land reference. A route is a set of positions in which it aims to position the cable with respect to the land reference. For example, if the cable 110 is discretized in N points Pi ... PN, the route can be given by N parametrized arcs (Pi, t) that correspond each to the position of a point Pi of the cable 110, in the reference terrestrial, depending on time. It is also possible, by approximation, to represent a route by means of a single parameterized arc, which then corresponds to the positions in the terrestrial reference of a point of the cable as a function of time, for example the center of the cable. In general, there is a route whose cable does not move away during its evolution in the water. Appropriately, the route is subject to a maximum speed value with respect to the land reference. Thus, there is a reference of low speed in the land reference whose cable never goes beyond the maximum value of deviation. The limitation of the maximum value of deviation combined with the fact that the route is subject to a maximum speed allows controlling the positioning of the device with respect to the part of the subsoil zone to be surveyed, and in this way obtain a better seismic density. It is referred to as the "desired" route since it is a route that the cable should ideally follow, but with respect to which deviations are authorized (within the limits authorized by the maximum deviation value), which reduces tensions Mechanics experienced by the cable and fuel consumption.

The method will now be described according to a first example of mode. In this first example, the cable evolves in a marine environment. The desired route comprises a position of the fixed land reference for a period of time. In other words, in this period of time the route is confused with a point that does not move in the terrestrial reference. It is then said that the cable is maintained during the period of time in almost stationary, even stationary ("substantially stationary") position, since the cable never deviates beyond the maximum value of deviation from the fixed position in question.

Indeed, it is said of a cable that evolves in an aquatic environment, which is "kept in almost stationary position" (respectively "stationary") if it evolves in water (for example according to a "route" as defined above) to remain almost stationary (respectively perfectly stationary) in the terrestrial referent (that is, the "absolute" referent). In other words, the projection of the cable on the seafloor never deviates beyond a predetermined value (i.e., maximum deviation value) of a position of the fixed land referent during a given period of time. The maximum deviation value can be linked to the dimensions of the cable. In one example, the deviation of the cable from the fixed point is calculated as the distance between the center of the cable (or any point of the cable) and the fixed point. The near stationary is then translated by a deviation preferably less than ten times the cable length, more preferably less than twice the length of the cable, and more preferably still less than the cable length, even at the half-length of the cable (These values can also be applied to other examples of the procedure). In general, the more limited the deviation is for a low maximum value, the more the density of shots made in the measuring station is distributed homogeneously on the ground. This allows collecting data that allows a good analysis of the part of the subsoil area to prospect below the measuring station in a shorter time, the extreme case being that of perfect stationarity.

By keeping cable 110 (or cables) in an almost stationary position, it increases its life as it experiences less stress than a cable that is continuously towed at a significant speed. Furthermore, cable 110, if provided with hydrophones and geophones, or vertical pairs of hydrophones, can be submerged more deeply than dragged cables, which protects it from the risk of accidents with other vessels, which limits the generation of Noises, especially from the waves. In this way, the cable is more protected the greater the depth 108 (that is, the distance from the surface 112 of the water) the more important. In addition, the device allows a more efficient and faster seismic survey, since measurements of the harmful effect of ghosts can be corrected. The cable is preferably submerged, between two waters (ie the cable is neither on the surface 112 nor resting on the bottom of the sea - where the ground may be uneven, which generates noise in the reception of the signal) to a depth that can be between 5 and 1000 meters, preferably between 5 and 500 meters, preferably between 10 and 300 meters, preferably between 20 and 200 meters, and more preferably still between 50 and 100 meters. The cable 110 is suitably provided with ballasting elements 104 intended to keep the cable submerged. The ballast allows the cable to maintain its sensibly constant depth and vary it in a controlled manner.

The cable 110 is suitably provided with tensioning means (i.e. exerting a tension force) symmetrically at its two ends, for example drones 102 as in the example of Figure 1. The drones 102 are of the type known per se., for example floating or semi-submersible with diesel propulsion or of the electrical type powered by a cable connected to a power source in a main ship. The drones 102 may include propulsion means (propeller) for towing and holding the cable 110 in tension, and more specifically for the central part carrying the sensors 106, is substantially horizontal, as is the case in Figure 1, and arranged to a constant depth 108 mentioned above. The device 100 can be designed to have zero or slightly positive buoyancy. Properly, the drones include, in addition, means of electrical connection with the respective cables for data communications and power, and seismic data recording means.

Keeping in almost stationary position requires less energy than towing, especially since the number of cables 106, their dimension and their mass are important. Preferably, the device 100 comprises between 10 and 50 cables 110, or between 15 and 30 cables 106, or also 20 cables 106. The cables have a length comprised between 1 and 20 km, preferably between 2 and 6 km (preferably approximately 4 km) or between 6 and 14 km (preferably approximately 8 km). In general, the configuration that allows a good study of the subsoil area of the perspective with fewer possible means, therefore the lower costs, is a configuration that comprises between 15 and 25 cables, preferably between 18 and 22 cables, or also preferably 20 cables, the cables having a length L such that L = k * d where d is the depth of the objective, ie of the most interesting subsoil region for prospecting, and k is a preference factor comprised between 0.8 and 1.5 and still more preferably substantially equal to 1.

Typically, in a measuring station, the device 100 is formed by several cables 110 that evolve in the water so as to remain in an almost stationary position substantially parallel to each other to form a grid above the field to be scanned. . The cables 110 can then be arranged inside the device 100 so that, if they are rectilinear, they substantially form waves. The reflected waves allow data collected by geologists to be collected. These operations in a single measuring station typically last several days, for example one week.

In a first case, the displacement of the seismic source comprises the tracking of several lines substantially perpendicular to the cable, the period of time during which the cable evolves almost stationary (even stationary in particular cases) substantially equal to the duration of the tracking of the lines. In other words, while the cable is held in a substantially fixed position with respect to the ground reference, the source pulls the waves along lines perpendicular to the cable. The points from which the source emits a wave thus constituting a grid of points above the part of the area to be surveyed. This allows optimal coverage of the part in question.

In another case, the displacement of the seismic source comprises the tracking of a line that is substantially perpendicular to the cable and that passes preferably at the level of a cable center, the period of time being substantially equal to the duration of the tracking of the line. In this case, during the period in which the cable remains almost stationary, a single line is therefore followed.

The route can then comprise other positions with respect to the land reference that corresponds to other parts of the area to be surveyed, each of the others being fixed during a respective period of time and the displacement of the seismic source can comprise the tracking of the line during the respective period of time, each respective period of time being equal to the duration of the line tracking. In other words, the cable is kept in a first almost stationary position. While the duration of holding in this first position, a first line of fire is followed by the source. The cable is then brought to a second, almost stationary position where it is held for a second period of time. During this second period, a second line of fire is followed by the source. And this is repeated, to obtain as in the first case a grid of points above the field under study with the same advantages. It should be noted that the line is always the one that is perpendicular to the cable and that passes preferably through its center. In this way, the movement of the source in reference to the cable tie, outside the periods in which the cable is not kept in an almost stationary position, consists of swings along this line.

The route may also include longitudinal displacements of the cable between the fixed positions of the land reference. These displacements allow moving the cable from an almost stationary position to another with less effort.

To maintain an almost stationary position with respect to the sea floor, a classic object such as a ship or an oil platform despite the currents, you can model its position with a point and control it respecting a reference position (ie position). absolute). Any deviation from the desired absolute position, given for example by GPS sensors, initiates a reaction of the propellers of the object to return it to its desired position, which the dimensions of the object allow to do without undue efforts.

Two referents can be considered: the reference "water" (or marine) in which the object sails, and the "absolute" referential, linked to the bottom of the sea or to the terrestrial reference. In the presence of a marine current that is the superposition of a constant current Vc and a circular tidal current ~ v ~ m, the water reference is displaced with respect to the absolute reference having as velocity vector: see = Ve + v "os ^ t) designating? the characteristic pulsation of the tidal current and t the time.

If the velocity vector with which the object is displaced with respect to the water reference is indicated by Voe, the object moves in the absolute reference with the velocity vector which is the sum of the two previous velocity vectors, namely: Voa = Voe + See = Voe + Ve + Vm * cos (üit).

For the object to be perfectly stationary in the terrestrial referent, its absolute velocity must be null, that is, voa = 0 - This implies: Voe = -vea = -ve - Vm * cos ((út) In other words, the object must travel in the water at an inverse speed of the speed of the marine current at time t considered. But with this, its route in the water reference is given by the integral of oe and the trajectory of the route is a complex curve illustrated in figures 2-4. If you see > Vm, the curve has the shape represented by figure 2. If Ve = Vm, the curve has the shape represented by figure 3. If Ve < Vm, the curve has the shape represented by figure 4. It is observed in these figures that, according to the current, the path of the object in the water may present loops and even inflection points.

In the framework of the procedure, in order to keep in an almost stationary position with respect to the terrestrial reference, the cable 110 or any other seismic cable, the cable 110 is evolved in the water, according to an almost stationary path (that is, the cable is evolved) 110 with a limitation of maximum deviation with respect to a desired route that is a fixed point of the land reference), the quasi-stationary path being limited by a maximum value of curvature.

The length of the cable 110 can be at least one hundred times greater than its transverse dimensions. The transverse resistance of the cable 110 is then considerably more important than its longitudinal resistance. Moving the cable 110 in the water according to its axis is relatively easy. On the contrary, the fact of navigating in the water perpendicular to its axis is extremely difficult. Typically, in the latter case, for a cable some kilometers long and 10 cm in diameter, the resistance would be several tens of tons at a speed in the water of 1 knot, which is too high. In addition, the resultant stresses on the cable 110 would produce stresses that would lead to its breaking.

Maintaining the cable 110 in perfectly stationary position with the known procedures would suppose a motorization and tensions exerted on the cable 110 too important for some types of marine current. For example, if it is considered that the current is such as that referred to in Figure 4, it would be necessary that each of the points of the cable follow a route in the water such as that of Figure 4. In particular, and as represented by the arrows in Figure 5, the ends A and B (and therefore all other points) of the cable follow at times in the water reference a path of strong transversal component (ie, perpendicular to the axis of the cable 110). This would give rise to the drawbacks mentioned above.

The proposed method of keeping in an almost stationary position therefore allows obtaining the advantages of keeping in an almost stationary position, namely the reduction of noise, the reduction of energy costs with respect to towing, the authorization to immerse the cable more deeply than when towed, while decreasing the energy costs and the mechanical stresses exerted on the cable 110.

It is now explained in more detail how a cable can be held in an almost stationary position for a given period of time.

For example, as illustrated in FIG. 6 in the case of a current vcvariable in intensity but constant in direction, the cable 110 is in the direction of the current and moves in front of the current according to the arrow 160 with an opposite speed in all directions. moment to that of the current. In this case, the cable 110 is perfectly stationary in the absolute reference. The cable 110 that evolves in the water along its axis, does not experience any resistance or overly important limitations. If the current is reversed while keeping the same direction, as shown in Figure 7, the cable 110 remains in the same direction but moves in the same reverse direction represented by the arrow 170.

The maximum curvature may depend on the length L of the cable 110. In this way, the maximum curvature may be a decreasing function of the length of the cable 110. For example, one may have a maximum curvature equal to q / L with q typically between 1/4 and 1. Such dependence allows a better seismic density.

The cable 110 is appropriately set in motion by one or the other of two drones, such as the drones 102 of Figure 1, each placed at one end A or B of the cable 110. This allows a quick and simple reversal of the direction of travel of the cable 110. In an instant of the displacement, for example, one of the two drones traces the cable 110 with the direction of the course while the other helps to maintain a minimum tension in the cable 110. In particular, the drone in the front part of the direction of displacement directs the cable 110 orienting itself and exerting a traction / tension more important than the rear drone. This prevents the cable 110 from leaving the path followed by the forward end of the cable 110 by lateral displacements. The rear drone pulls in the opposite direction on the axis of the cable 110 to exert a minimum tension typically greater than 50 kg and less than 500 kg, which prevents compressions of the cable 110. The two drones can pull alternately. Typically, the traction stage from one drone to another can be subject to the tension of the cable. It occurs, for example, as soon as the measured voltage is lower than a predefined value. Drones can be orientable within narrow limits Appropriately, the method comprises, before the stage of evolution of the cable, the steps of: provision of prediction values of the marine current; of determination of a theoretical path of the cable in the water that corresponds exactly to the desired route, based on the prediction values of the marine current (a route in the water is determined that guarantees the exact tracking of the desired route); of determining a real path of the cable in the water by approximation of the theoretical path minimizing a deviation between the real path and the theoretical path, the minimization being limited by the maximum curvature; of the evolution of the cable that includes the displacement of the cable in the water according to the actual travel. This allows management of energy consumption and management of the mechanical tensions in the cable that are optimal.

In one application in the first example, which can be generalized to all the described examples of the method, the method of maintaining the quasi-stationary position of the cable 110 comprises a step of supplying prediction values of the marine current. These values can be supplied, for example, by specialized institutes, or obtained in real time by means of current measurement. In this example, the method also includes the determination of a stationary path with respect to the terrestrial reference in the marine environment based on the prediction values of the marine current. For this, the predicted current can be integrated in time to provide the stationary path of a point reference. The method then comprises a step of determining the almost stationary path by approaching the stationary path with respect to minimizing a deviation between the almost stationary path and the stationary path. The minimization is limited by the maximum curvature value. This can be done by filtering (that is, smoothing) the stationary route, having as a limitation the fact that the filtered (ie smoothed) path, ie the almost stationary path, must have at each instant a curvature lower than the maximum curvature value. This smoothing may comprise an interpolation, for example polynomial, of the stationary path, or also a regression of the stationary path. The cable then moves in the marine environment according to the almost stationary travel thus determined.

This example can be applied by real-time references, such slogans can be deduced from the currents and transmitted to the drones. The direction of the drones is given by the almost stationary route.

As already explained, it is possible by integration to calculate the distance in the water of a virtual point object (hereinafter referred to as the reference (or objective) R) and that it would remain in stationary absolute position, for example by conventional means. It is not possible to realize that all the points of the cable 110 follow the corresponding stationary route, if it is too complex, for the reasons mentioned above. But the cable 110 can follow without any major limitations a smooth path, which especially avoids the loops and the inflection points. The smoothing can be carried out in a length comprised between 0.5 and 3 times the length of the cable 110. The result of the smoothing is presented in figure 8, where the continuous line indicates the trajectory (that is, the set of the P positions) of the stationary travel. R, and the mixed trace indicates the trajectory of the almost stationary path determined by approximation of the stationary path.

The cable 110 at ends A and B thus follows the almost stationary path, and is seen in two different positions u1 and u2 in FIG. 8. As is the case in FIG. 8, over time, the orientation of the cable 110 can change since the path of the almost stationary travel can itself be curved in the long term. In order to leave the cable 110 with the possibility of turning on itself in the long term, the deviation between the almost stationary travel and the stationary travel can be modeled by the deviation between a point M of the cable and its reference position RM (the position that would have had if followed a perfectly stationary route). This point M can be any point of the cable 110, for example its center. The choice of the center is the one that gives the best seismic density. The minimization of the deviation between the almost stationary travel and the stationary travel can then consist in integrating, for the global displacement, the distance between M and RM.

In the configuration of figure 9, which shows the trajectory of the almost stationary path in mixed stroke and the trajectory of the stationary stroke in a continuous line, in an out-of-phase manner for reasons of clarity, the reference point RM advances in the water at the speed VRMe ( inverse of the actual current). The end B drone pulls the cable 110 with the speed in the VBe water. which is the projection of the vector VRMe in the filtered path. The drone is therefore controlled in speed so that M remains as close to RM. The drone is also controlled in course by the definition of the filtered route. The propeller in A can be inactive or ensure minimum tension, as indicated above.

In this way, the M point always remains at a minimum distance from RM. In the absolute reference, this distance is the same since the two points RM and M experience the same current. In this way, according to this principle, the selected point M of the cable 110 remains at a minimum distance from the pointed absolute position. On the other hand, the propeller B, that sails in a smooth route, does not impose important limitations to the cable 110.

The drone in B can stop towing. The drones A and B can interchange their functions, guaranteeing since then the drone in A the control with the same principle of control in speed and being B inactive or guaranteeing a minimum tension. This allows, if the VRMe projection in the almost stationary path is canceled and the sign changes, reverse the direction of traction, as shown in figures 10 and 11.

With such a procedure, the velocity vector of the ends A or B always has a smooth direction: there are no major course changes resulting in the absence of limitations in the cable 110 thus displaced. The speed module is given by the projection of the reference speed (inverse of the true current) in the path of the filtered path: the cable 110 therefore remains at a minimum distance from this reference.

As illustrated by figures 12 and 13, this is equally true in the absolute referent (terrestrial). Figure 12 presents in the water reference three successive positions (u1, u2, u3) of the cable during the displacement. Figure 13 presents these three successive positions (u1, u2, u3) in the absolute referent. The selected point (for example the center) remains at a minimum distance from the fixed point RM. The absolute position of M given by the M-RM vector can fluctuate but only according to the high frequency component of the current (tidal component, for example). In this way, he describes a small closed curve. The orientation of the cable 110 changes, on the other hand, according to the very low frequency (unfiltered) component of the current.

With reference to figures 14 to 17, the method can alternatively, for example in the absence of current prediction, comprise a stage of real-time supply of a target position (the target position being the stationary position corresponding to the fixed position of the route in the presented case, and a desired objective position deduced from the desired route according to the instant t in the general case of all the described examples) and a stage of displacement in the direction of the objective position, the displacement being limited for the maximum curvature value. The two stages of supply and displacement are then repeated. This allows the cable 110 to be held in almost stationary position despite the absence of predictions. Typically, the repetitions come at regular stages. Alternatively, the stages can be variable and depend on the current. At each stage, the deviation from the stationary position is observed, and it is reduced while taking into account the maximum curvature value, which makes it possible to avoid motorization or limitations that are too important. The interval between two stages typically lasts from a few seconds to a few tens of minutes, preferably between 1 and 10 minutes.

The ends A and B of the cable 110 can be equipped with an absolute positioning (GPS, for example), as well as classical speed sensors relating to water, bearing, tension on the object. The point M of the cable can also be equipped with a measurement of speed relative to water and bearing (magnetic compass, flux gate, for example). On the other hand, the assembly is positioned by known relative means (acoustic, magnetic compasses) possibly adjusted in the GPS position of the propellers (known procedure itself).

The objective position of stationarity can be the position in the absolute referent of the reference RM of M. For a point M of the cable 110 any, for example the center, is therefore the position in the water reference which should have to be stationary As illustrated in FIG. 14, the displacement in the direction of the target position RN may comprise a step of projecting the target position RM on the cable 110 at a point P and a step d calculating a maximum course limited by the maximum value of curvature and the speed of the cable in the water. The projection step can comprise the determination of a line 130 perpendicular to the cable 110 passing through RM. This perpendicular is called the reference line and cuts the cable 110 at point P.

The reference line 130 serves to control the speed that the method can comprise. For example, as is the case in Figure 14, if M is delayed with respect to reference line 130 and point P, the drone placed in B accelerates to return M to P in the following instants. The control loop uses classical techniques known per se. In short, the control loop can take into account the deviation observed between two stages and adapt the speed in function.

The procedure may also comprise control of the heading of the drone placed at B. This control may not involve only the target position RM. The vector AB represents the direction of traction by the drone B of the cable. To approach the objective point RM in the case of figures 14 and 15, drone B can turn left. As the course changes per unit time are limited by the maximum curvature, no resistance or too strong limitations are imposed on the cable 110. The maximum value of this course change during a given time interval which allows for example limiting the curvature of the trajectory can be deduced from the following known formula: Da Ve dt #min where Ve is the speed in the water of the cable, Rmin the minimum radius of curvature (inverse of the maximum curvature) and dt the time interval considered.

The speed in the water of the cable can be known by the means already described, for example a slide or a Doppler sonar placed in each drone and in a number of other points of the cable between which the selected point M. You can also use the measurement of the tensile force, or the speed of rotation and the stage of the helix of the drone if applicable. Through a hydrodynamic model of the whole system.

The speed and heading increases of the propeller can be given by a control loop where the objective is to bring the point M closer to the reference line in priority (for speed) and to the fixed point RM respecting the limitations mentioned above of changes of course. Figure 15 shows the old velocity ~ vei and the increased velocity ve2 respecting these conditions.

For a state in which the point M has exceeded the reference line as shown in figure 16, the drone in 6 decreases while it turns to the left towards the objective position with the same limitations as before.

By decreasing its speed in the water, a moment can be found in which the speed becomes null (without any traction effort) and it can not be admitted that it becomes negative, that is to say that the propeller B starts to push back the cable. At that moment the order is given to the propellers A and B to exchange their functions, B then becoming active and guaranteeing a traction in the opposite direction with a freedom of maneuver in course according to the same limitations. In the case where the drone ensures a minimum voltage, the transfer of the functions is carried out as soon as the tensile stress B becomes lower than the minimum voltage required in the object.

Keeping cable 110 almost stationary can comprise a first phase of maintaining the cable according to the procedure of the example in which a good current prediction is available, and a second phase of prediction of the cable according to the real-time procedure in the that a good prediction of the current is not available. Such a procedure makes it possible to adapt the retention of the prediction data.

Especially, during the second phase, current data can be recorded and serve as a basis for the prediction of the current. You can then enter the first phase. Furthermore, during the first phase, the deviation from the theoretical stationary position (with the actual marine current) can be controlled, for example by GPS. Indeed, current predictions are not necessarily perfectly accurate, and there may be a long-term deviation between the determined stationary path and the theoretical stationary path. As soon as a deviation threshold is reached, one can then enter the first phase. In this way, it is generally possible to switch between the first phase and the second phase depending on the prediction values available and / or a deviation from the stationary position.

Again, all the necessary data for the different controls can proceed, for A and B (and even the intermediate points) of the GPS positions, of the voltage applied by the propellers, the slides or sonar-Doppler, of the angles of rudder, et cetera. This data nourishes a computer program that, depending on the given objective point, will communicate to the thrusters the necessary speed and rudder commands.

The method can also be implemented according to a second example in which the teachings of the first example are applied. This second example differs from the first example because the desired route is a straight line. In this way, it is desired that the cable travel according to a straight line with respect to the land reference. The procedure of the second example allows longitudinal sweeping of a subsoil area to be surveyed. In this example, the displacement of the seismic source can comprise several portions according to a line substantially perpendicular to the cable and preferably passing to the level of a center of the cable. In the reference linked to the cable, the seismic source performs oscillating movements according to this line. A grid of wave emission points extending in accordance with the length of the cable is thus established.

The method can also be implemented according to a third example in which the teachings of the first example are applied. This third example differs from the first example because the desired route comprises the lateral displacement of the cable with respect to the terrestrial reference. In this example, the displacement of the seismic source may comprise the path of lines substantially parallel to the cable, the lines being between two central cables of the device. In this way, a wave emission point grating extending in a direction transverse to the length of the cable is established. This case may correspond to a drift situation of the cable according to the constant component of the current.

The second and third examples of the process have advantages similar to the first example. As the desired route is subject to a maximum speed value with respect to the land reference, the seismic source has time to sweep the area to produce the waves and the seismic density is only little affected. The maximum speed value is less than 1 knot, preferably less than 0.5 knot, preferably less than 0.2 knot. However, you can sweep a larger area and avoid the discontinuity of point measurements while taking advantage of the constant component of the current so that the cables move with respect to the land reference, even if the route has the direction of this component.

The method can, in general, comprise a step of measuring the velocity in water. This measurement can serve as a basis for other stages of the procedure. For example, the measured speed can limit the maximum curvature and / or course changes. The measured speed can be acquired with the help of measuring means. It may be the speed either at the level of the drones, or preferably at the ends of the submerged cable, either along the cable or at its center. Surface currents may be different from currents at 100 m depth for example. Thus, in the case where a disposition of the surface currents is available, but the cable is submerged, the measured speed can be used to adapt the predictions to the depth at which the cable is located.

In this way, a computer program may contain instructions for the implementation of the procedure described above. This computer program can be recorded on a classic medium, such as a CD-ROM, a hard disk, or other types of memory, possibly fractionated.

The seismic survey device may comprise one or more cables provided with sensors, such as the cable 110 and a calculation unit for the evolution of the cable 110 in the water, limited by a maximum curvature value of travel in the water and by a maximum deviation value with respect to a desired route in the land reference, the route being appropriately subjected to a maximum speed value with respect to the land reference. In particular, the device is specially adapted for the method described above.

Such a device has a longer lifespan than the seismic survey systems of the prior art since fewer cable limitations are inflicted. In addition, the device consumes less energy. The cable can also be provided with two drones, each connected to one end of the cable. The cable can also be provided with ballast.

The device may comprise several cables adapted to be kept in an almost stationary position in a substantially parallel manner, one with respect to the other, each in the manner described above. The cables are, however, preferably able to evolve freely with respect to each other, so that no means of attachment such as paravans, or else to hinder the longitudinal movement of the device.

Figure 18 shows a top view of the device 100 including a plurality of cables 110, which are substantially parallel. Likewise, Figure 18 shows the monitoring by a seismic source 212 of a line 200 substantially perpendicular to the cables 110 and that preferably passes substantially to the level of a center M of the cables. The line 200 includes points 210 from which the seismic source 212 makes shots during the tracking of the line 200. The points 215 represent (in the terrestrial reference) previous shots during the tracking of line 200, the direction of travel being of the cables according to arrow 216 in the land reference. Figure 19 shows an example of a cable displacement control loop. In this example, the cable is subjected to a marine current and the displacement of the cable in the water compensates the current. According to the example, the displacement of the cable is subject to current. In the example, the cable is also provided with two drones each connected to one end of the cable, indicated as "drone 1" and "drone 2" in figure 19. The displacement of the cable follows a control loop comprising a algorithm 199 that takes as inputs the following parameters: • The position (190 and 191) of each drone, obtained by GPS, • The desired target position 192 obtained from the desired route, • A calculated position 193 from the center of the M cable, obtained by GPS, acoustics, depth sensors and compasses, • A speed in the water 194 of the cable measured by Doppler probe, • A prediction 195 of the marine current, • A current heading 196 of each drone, and • A current voltage 197 exerted by each drone on the cable.

The algorithm 199 provides as output 200 a new course of each drone and a new voltage to be exerted by each drone on the cable. The algorithm can be selected from algorithms known to the person skilled in the art.

Claims

1. - Seismic survey procedure in aquatic environment with the aid of a device comprising at least one seismic cable (110) provided with sensors (106) and at least one mobile seismic source, comprising the steps consisting of move the cable in the water by means of two drones (102) each placed on one end of the cable and that keep the cable in tension, minimizing the displacement of the cable the deviation of the cable with respect to a desired route in the terrestrial referent, being, in addition , the displacement of the cable limited by a maximum curvature value of travel in the water and, simultaneously, move the seismic source in a referent linked to the cable, emit waves by the seismic source, and capture reflections of the waves by the cable.

2. - Method according to claim 1, in which the cable is subjected to a marine current and the displacement of the cable in the water compensates the current.

3. - Method according to claim 1 or 2, in which the route is subjected to a maximum speed value with respect to the land reference, and the maximum speed value is less than 1 knot, preferably less than 0.5 knot, preferably less than 0.2 knot.

4. - Method according to any of claims 1 to 3, wherein the device comprises several cables substantially parallel to one another during the procedure.

5. - Method according to claim 4, wherein the device comprises between 15 and 25 cables, preferably 20 cables, the cables having a preferred length of between 1 and 20 km, preferably between 2 and 6 km, preferably about 4 km, or between 6 and 14 km, preferably between 8 km, the cables being separated from each other by a distance of between 100 and 1000 m, preferably between 200 and 800 m, preferably between 350 and 450 m.

6. - Method according to any of claims 1 to 5, wherein the desired route comprises a position of the fixed land reference for a period of time.

7. - Method according to claim 6, in which the displacement of the seismic source comprises the tracking of a line perpendicular to the cable and which preferably passes substantially at the level of a cable center, the period of time being substantially equal to that of the cable. duration of line tracking.

8. - Method according to claim 7, wherein the route comprises other positions of the land reference, each position being fixed during a respective period of time and the displacement of the seismic source comprises the tracking of the line during the respective time period , each respective time period being substantially equal to the duration of the line tracking.

9. - Method according to claim 8, wherein the route further comprises longitudinal displacements of the cable between the fixed positions of the land reference.

10. - Method according to claim 6, in which the displacement of the seismic source comprises the tracking of several lines substantially perpendicular to the cable, the period of time being substantially equal to the duration of the tracking of the lines.

11. - Method according to any of claims 1 to 5, wherein the desired route is a straight line.

12. - Method according to claim 11, in which the displacement of the seismic source comprises several tracks of a line that is substantially perpendicular to the cable and that preferably passes substantially at the level of the center of the cable.

13. - Method according to any of claims 1 to 5, wherein the route comprises the lateral displacement of the cable with respect to the land reference.

14. - Method according to claim 13, in which the displacement of the seismic source comprises the path of lines substantially parallel to the cable, the lines being between two central cables of the device.

15. - Method according to any of claims 1 to 14, in which the maximum curvature depends on the length of the cable and the speed in the water.

16. - Method according to any of claims 1 to 15, comprising measuring the speed in water.

17. - Method according to any of claims 1 to 16, in which at an instant of evolution, one of the two drones puts the cable in motion, with direction of course while the other drone maintains a minimum voltage in the cable .

18. - Method according to any of claims 1 to 17, in which the two drones put the cable in motion alternately.

19. - Method according to any of claims 1 to 18, comprising, before the step of moving the cable, the steps of: supply of prediction values for marine current; determination of a theoretical path of the cable in the water that exactly corresponds to the desired route, based on the prediction values of the marine current; determination of a real path of the cable in the water by approximation of the theoretical path minimizing a deviation between the real path and the theoretical path, the minimization being limited by the maximum curvature; being the displacement of the cable in the water the real journey.

20. - Method according to any of claims 1 to 18, wherein the step of moving the cable in the water comprises the sub-steps of: provision in real time of a desired objective position from the desired route; displacement in the direction of the target position, the displacement being limited by the maximum curvature; repeating the two sub-stages of supply and displacement.

21. - Method according to claim 20, in which the displacement in the direction of the target position comprises a step of projecting the objective position on the cable and a step of calculating a maximum course limited by the maximum curvature value and the speed of the cable in the water.

22. - Procedure according to any of claims 19 to 21, comprising: a first phase of seismic prospecting according to claim 19, and a second phase of seismic prospecting according to claim 20 or 21.

23. - Method according to claim 22, comprising a switching between the first phase and the second phase depending on the prediction values available and / or a deviation from the target position.

24. - a seismic prospecting device comprising: at least one cable (110) provided with sensors (106) and two drones (102) each placed on one end of the cable to put the cable in motion and keep it in tension; a calculation unit to determine the displacement of the cable in the water, minimizing the displacement of the cable the deviation of the cable with respect to a desired route in the terrestrial reference, being, in addition, the displacement of the cable limited by a value of maximum curvature of travel in water.

25. - The device according to claim 24, wherein the cable is also provided with ballast (104).

26. - The device according to claim 25, wherein in one instant of the displacement, one of the two drones puts the cable in motion with direction of course while the other drone maintains a minimum tension in the cable, being the two drones able to put the cable in motion alternately.

27. - The device according to any of claims 24 to 26, further comprises means for measuring the speed in the water of the cable.

28. - The device according to any of claims 24 to 27, wherein the cable has a length preferably of between 1 and 20 km, preferably between 2 and 6 km, preferably of approximately 4 km or between 6 and 14 km, preferably approximately 8 km.

29. - The device according to any of claims 24 to 28, comprising several cables capable of being set in motion freely with respect to each other.

30. - Procedure for deploying in an aquatic environment a device comprising at least one seismic cable (110) provided with sensors (106), comprising a step consisting of moving the cable in the water by means of two drones (102) placed at one end of the cable and that keep the cable in tension, minimizing the displacement of the cable the deviation of the cable with respect to a desired route in the terrestrial reference, being, in addition, the displacement of the cable limited by a value of maximum curvature of travel in the water.

31- Procedure according to claim 30, wherein the desired route comprises a fixed land reference position for a period of time.

32. - Method according to any of claims 30 or 31, in which the cable is subjected to a marine current and the displacement of the cable in the water compensates the current.

33. - Method according to claim 32, in which the displacement of the cable is subjected to the current.

34. - Method according to any of claims 1 to 23 or 30 to 33, in which the displacement of the cable follows a control loop comprising an algorithm that takes as inputs one position of each drone, a desired target position from the desired route, a calculated position of the center of the cable, a speed in the water of the measured cable, a prediction of the marine current, a current course of each drone and / or a current tension exerted by each drone on the cable, and providing the algorithm as output a new course of each drone and / or a new voltage to be exerted by each drone on the cable.